The disclosure relates to a method for controlling the activation of a hydraulic vehicle brake system, which has a brake booster, of a motor vehicle which has an electric drive motor. Furthermore, the disclosure relates to an electromechanical brake booster designed specifically for the method described herein. There is no differentiation here between open-loop control and closed-loop control (open-loop control within the context of the disclosure also comprising closed-loop control, and vice versa). Brake activation refers to the activation of the vehicle brake system by a vehicle driver using muscle force applied by a foot or hand. The motor vehicle may be an electric vehicle driven exclusively by one electric drive motor or else by a plurality of electric drive motors. In particular, the disclosure is provided for a hybrid vehicle which has an internal combustion engine and, in addition, one or more electric drive motors. In order to recover energy, for braking, the electric drive motor can be operated as a generator. A driving torque for driving the electric drive motor as a generator decelerates the motor vehicle in the form of a braking torque. The electric current generated by the generator mode is stored in a battery and is available for driving the motor vehicle by means of the electric drive motor.
A braking action of the electric drive motor in the generator mode is dependent, inter alia, on the vehicle speed and, for example, also on the state of charge of the battery, and, when the battery is fully charged, the braking action is virtually zero. Even at a low speed, the braking action is low and is reduced to zero as the vehicle stops. The motor vehicle therefore has to be braked jointly with the hydraulic vehicle brake and the electric drive motor in the generator mode, wherein the portion which the hydraulic vehicle brake system has to contribute to the braking fluctuates between zero and one hundred percent. The control of the portions contributed to the braking action by the electric drive motor in the generator mode and by the hydraulic vehicle brake system is referred to as “blending”.
It is possible to leave the “blending” to the vehicle driver, i.e. the latter adapts his/her muscle force to activating the braking action of the electric drive motor in the generator mode.
An indispensable demand imposed on the braking of a motor vehicle with an electric drive motor in the generator mode is that the braking distance must not be extended.
In electrohydraulic vehicle brake systems, the blending is possible in a comparatively simple manner without being noticed by the vehicle driver. Electrohydraulic vehicle brake systems are power-brake systems in which the energy required for brake activation does not originate from the muscle force of a vehicle driver but exclusively from an external energy supply device, and the brake pressure is generated by a hydraulic pump. The vehicle driver prescribes a desired value for the brake force at a brake pedal.
Hydraulic vehicle brake systems are known per se and will not be explained in more detail here. Hydraulic vehicle brake systems are also known with wheel slip control, said systems having, for each wheel brake, a brake pressure buildup valve and a brake pressure reduction valve, with which a wheel brake pressure and therefore a braking force of the particular wheel brake can be modulated, i.e. can be controlled by open-loop and closed-loop control, for the purpose of slip control. The wheel brake pressure can either be reduced in order to avoid locking of a vehicle wheel during braking, or a wheel brake pressure can be built up in order to avoid or to limit spinning of a driven vehicle wheel during starting up and/or accelerating, and also individual vehicle wheels can be braked in a specific manner in order to reduce a tendency to skid in critical driving situations.
Vacuum brake boosters, the structure and function of which is known and therefore will not be explained here, can be considered to be customary nowadays. Electromechanical brake boosters which generate an auxiliary force, for example electromotively or electromagnetically, for activating a master brake cylinder are also known. For example, an electromechanical brake booster with a solenoid or a linear motor for generating the auxiliary force for brake activation is known from laid-open application DE 100 57 557 A1. In the same manner as a vacuum brake booster, an electromechanical brake booster, in addition to a muscle force exerted by a vehicle driver, also exerts the auxiliary force, which said brake booster generates, on a master brake cylinder of the vehicle brake system in order to build up a brake pressure.
The disclosure can also be used for motor vehicles or other vehicles which have a generator which is used for braking without being an electric drive motor.